I have never had any problem with China Postal services, and they seem to more or less reliable like the postal services in your country.
Your mail or parcels or whatever will be delivered to the school when they arrive. Have you ever thought that they might have "disappeared" there? I won't trust any school officials if things were stolen from you before.
The pity is you probably can't prove anything, so they can get away with it if it was really them taking your stuff.

Anyway, as i said, has anyone else encountered problems with lost ( more likely stolen) items in the post.